/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.xml;

import java.io.File;
import java.io.IOException;
import java.net.URL;
import java.net.URLDecoder;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

import org.springframework.core.io.Resource;
import org.springframework.core.io.ResourceLoader;
import org.springframework.lang.Nullable;

/**
 * EntityResolver implementation that tries to resolve entity references
 * through a {@link org.springframework.core.io.ResourceLoader} (usually,
 * relative to the resource base of an ApplicationContext), if applicable.
 * Extends {@link DelegatingEntityResolver} to also provide DTD and XSD lookup.
 *
 * <p>Allows to use standard XML entities to include XML snippets into an
 * application context definition, for example to split a large XML file
 * into various modules. The include paths can be relative to the
 * application context's resource base as usual, instead of relative
 * to the JVM working directory (the XML parser's default).
 *
 * <p>Note: In addition to relative paths, every URL that specifies a
 * file in the current system root, i.e. the JVM working directory,
 * will be interpreted relative to the application context too.
 *
 * @author Juergen Hoeller
 * @see org.springframework.core.io.ResourceLoader
 * @see org.springframework.context.ApplicationContext
 * @since 31.07.2003
 */
public class ResourceEntityResolver extends DelegatingEntityResolver {

    private static final Log logger = LogFactory.getLog(ResourceEntityResolver.class);

    private final ResourceLoader resourceLoader;


    /**
     * Create a ResourceEntityResolver for the specified ResourceLoader
     * (usually, an ApplicationContext).
     *
     * @param resourceLoader the ResourceLoader (or ApplicationContext)
     *                       to load XML entity includes with
     */
    public ResourceEntityResolver(ResourceLoader resourceLoader) {
        super(resourceLoader.getClassLoader());
        this.resourceLoader = resourceLoader;
    }


    @Override
    @Nullable
    public InputSource resolveEntity(String publicId, @Nullable String systemId) throws SAXException, IOException {
        InputSource source = super.resolveEntity(publicId, systemId);
        if (source == null && systemId != null) {
            String resourcePath = null;
            try {
                String decodedSystemId = URLDecoder.decode(systemId, "UTF-8");
                String givenUrl = new URL(decodedSystemId).toString();
                String systemRootUrl = new File("").toURI().toURL().toString();
                // Try relative to resource base if currently in system root.
                if (givenUrl.startsWith(systemRootUrl)) {
                    resourcePath = givenUrl.substring(systemRootUrl.length());
                }
            } catch (Exception ex) {
                // Typically a MalformedURLException or AccessControlException.
                if (logger.isDebugEnabled()) {
                    logger.debug("Could not resolve XML entity [" + systemId + "] against system root URL", ex);
                }
                // No URL (or no resolvable URL) -> try relative to resource base.
                resourcePath = systemId;
            }
            if (resourcePath != null) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Trying to locate XML entity [" + systemId + "] as resource [" + resourcePath + "]");
                }
                Resource resource = this.resourceLoader.getResource(resourcePath);
                source = new InputSource(resource.getInputStream());
                source.setPublicId(publicId);
                source.setSystemId(systemId);
                if (logger.isDebugEnabled()) {
                    logger.debug("Found XML entity [" + systemId + "]: " + resource);
                }
            }
        }
        return source;
    }

}
